Distribution Of Redox Sensitive Elements In A Sediment Core From Seasonal Low-oxygen Zone Of The Pearl River Estuary And Its Paleo-environmental Implications

Changes in dissolved oxygen in estuaries and coastal zones are associated with the reproduction of marine organisms and control the deposition and recirculation of metal elements in sediments.In recent decades,bottom water hypoxia(<2-3 mg/L)has gradually developed into a global marine ecological and environmental problem,thus attracting great attention from domestic and overseas scholars.In recent years,a growing number of studies have found the occurrence of seasonal low oxygen zone,up to approximately 1500 km²,in bottom waters of the Pearl River Estuary.As transition zones between land and ocean,the development of hypoxia in estuaries is mainly controlled by physical processes such as water column stratification and biological oxygen consumption that linked to eutrophication.Long time-series environmental datasets from estuarine and coastal environments can provide key information about natural,climate induced variability and human influences that are necessary for the sustainable management of coastal habits.However,there are few available datasets worldwide.Historical records can also be useful to reconstruct environemtal change in the past.A variety of biological and geochemical proxies derived from sediment cores have been used to reconstruct the development of eutrophcation and hypoxic conditions over time.In this thesis,a 19 m-long sediment core(WS)from the current seasonal low oxygen zone of the Pearl River Estuary was collected Core chronologies are obtained using ²¹⁰Pb and AMS ¹⁴C dating techniques.In order to provide a long-term perspective on changes in terrestrial inputs associated with human disturbance and the corresponding sedimentary responses,we mainly focused on the past 2000 years.Sedimentary Ti content,Al/K ratio,heavy metal(Cu,Pb)indicators and organic matter source analysis were used to reveal changes in terrigenous inputs,while the relationship between primary productivity and terrigenous nutrient input was combined to reflect the effect of human activities on estuarine environment.Based on the results of terrigenous source analysis,the redox sensitive elements(Fe-Mn-Mo)records were used to reconstruct the evolution process of redox status of sedimentary environment in the Pearl River Estuary during the last 2 millennium.In addition,the response of redox conditions in estuarine coastal zone waters to natural climate variability and human activities are discussed.The results indicate that human activities played a key role in shaping the evolution of sedimentary environments during the last 2000 years.As the human population rises in the Pearl River basin over last 2 millennium,intensified anthropogenic activities has led to an increase in nutrient delivery to the estuarine coastal zone,which consequently contributed to the increase in marine primary productivity.The variations of organic matter sources in the Pearl River estuary in different periods were analyzed by using a two-end-member mixing model based on sedimentary TOC/TN ratio:the relative importance of terrestrial OM source to total organic carbon pool in WS core was relatively high,but decreased gradually,while the absolute content showed fluctuating variability.In contrast,the relative proportion and absolute content of marine organic matter exhibited a clear upward trend,especially in the last 1000 years.The results of mineral composition analysis revealed the hydrodynamic processes in the Pearl River Estuary to some extent,and the differences in mineral composition and distribution show the relative enhancement of hydrodynamic processes during typical warming periodfeatures.The enrichment mechanisms of sedimentart RSEs show significant differences.In the present study area,Cu,Pb and V are directly or indirectly influenced by human activities,and theentry entered sediment via ferromanganese oxide adsorption and organic matter complexation.Combined with the analysis of terrestrial input indicators and organic matter source identification,the enrichment patterns of Cu,Pb and V in the Pearl River Estuary are not effective indicators of the occurrence of low oxygen events.Correlation analysis among redox-sensitive elements indicated that Mo enriched in the sediments mainly through the oxidative adsorption of Fe and Mn.Based on Fe-Mn-Mo covariance as an indicator to judge the palaeo-redox variation,the degree of redox in the Pearl River Estuary waters could be divided into two stages in the last2000 years.(i)a progressive decrease in sediment oxidation intensity,but an increase in reduction intensity during the last 2 millennium;and(ii)the aggracation of oxidized conditions during three typical warm periods.We further attempt to clarify the response mechanism of the evolution of hypoxia in the water column of the Pearl River Estuary under the combined stress of natural climate change and human activities.In South China,the East Asian summer winds have gradually weakened over the past 2000 years,and human activities during the same period will lead to increased nutrient input to the water column and promote rapid growth of marine primary productivity.In this context,the effective degradation of marine organic matter has led to an overall trend of continuous weakening of the oxidative properties of seawater in the Pearl River Estuary.On the contrary,the relative increase in precipitation during the three warm periods of Roman Warm Period,Medieval Warm Period and Modern Warm Period shortened the retention time of estuarine water bodies,which was not conducive to the growth and reproduction of oxygen-degrading aquatic phytoplankton,and the content of sea-sourced organic matter decreased.The decrease in chemical oxygen consumption during organic matter degradation led to a transient increase in the oxidative nature of the water column.From the above studies,it can be seen that the increase in nutrient input from terrestrial sources due to human activities and the changes in hydrodynamic conditions regulated by natural climate have combined to form the current trend in the evolution of the redox status of the water column in the Pearl River Estuary.